1. Field of the Invention
The present invention relates to a manufacturing method of multi-layer printed boards and double-sided printed boards used for hybrid ICs, multi-chip modules, chip-size packages, etc., and also concerns green sheets suitable for manufacturing the multi-layer printed boards and double-sided printed boards, and the manufacturing method of the green sheets.
2. Description of the Related Art
In recent years, the development of low-temperature sintered glass multi-layer printed boards has made it possible to use low-melting-point metals (such as gold, silver, copper, silver-palladium and mixtures thereof) having small wiring resistivities as conductor materials for multi-layer printed boards.
However, in these multi-layer printed boards, a green sheet (having ceramics and an organic binder as its main components), which is normally used as the board material, is subjected to a volume shrinkage of 10 to 20% by the firing process, and during the firing process, the shrinkage does not necessarily take place uniformly in respective directions. This results in a reduction in the dimensional precision, thereby causing degradation in the yield.
Such irregularities in the rate of the volume shrinkage can be reduced, for example, by the following method as disclosed in Japanese Laid-Open Patent Application No. 257553/1991 (Tokukaihei 03-257553). In this method, sintering resistant green sheets are stacked on the surface and the rear surface of a green sheet, and this laminated body is fired so that shrinkage occurs only in the thickness direction without virtually any shrinkage on the X-Y plane; thus, the above-mentioned irregularities can be reduced.
The application of such a non-shrinkage manufacturing process makes it possible to reduce irregularities in the rate of volume shrinkage and also to simultaneously fire the green sheet and the conductor wiring such as metal foil wiring.
In the non-shrinkage sintering method, in most cases, conductor paste is printed and formed into the shape of a wiring pattern, and this is simultaneously fired as the green sheet is fired so that the wiring is formed.
However, when the formation method of the wiring using the conductor paste is applied to the non-shrinkage sintering method, the following problem arises: The laminated green sheet that has no shrinkage in the X-Y direction prevents the sintering of the conductor paste, resulting in a high value in the conductor resistivity of the wiring.
In order to solve such a problem, metal foil is used as the wiring, and after the wiring of the metal foil has been formed on a green sheet, the green sheet is subjected to a laminating process, and then fired.
With respect to the method for forming the metal foil wiring on a green sheet, for example, Japanese Laid-Open Patent Application No. 242644/1998 (Tokukaihei 10-242644) has disclosed a transferring method using a thermal separation sheet. Here, in this transferring method, patterning is made by carrying out a wet etching process on metal foil affixed to the thermal separation sheet; however, since the thermal separation sheet lacks resistance to an etchant used for the wet etching, it is virtually difficult to form the pattern on the green sheet by using the transferring method.
In order to solve this problem, the applicant of the present invention has devised a method in which wiring is formed by directly forming a metal foil pattern on a green sheet by means of a subtractive method in which the wet etching method is used. However, although it enables the formation of the metal foil pattern, the wet etching method has a problem in which the etchant used in the patterning process gives damages to the green sheet, preventing the green sheet from being finely fired.
Therefore, an object of the present invention is to provide a green sheet having an etching resistant property.
Another object of the present invention is to provide a multi-layer wiring board having a low wiring resistivity without causing any problems in its manufacturing process.
In order to achieve the above-mentioned objects, the present invention provides a green sheet which mainly contains a ceramics material in powder and a binder, the binder containing an acrylic resin having no polar group.
First the present invention provides a green sheet containing a ceramics material in powder and a binder, wherein the binder contains an acrylic resin having no polar group.
Secondly the present invention provides a manufacturing method for a green sheet comprising the steps of: manufacturing a green sheet including the binder containing the acrylic resin having no polar group and the ceramics material in powder; and after having placed a conductor layer on the green sheet and then a mask on the conductor layer, forming a wiring pattern by patterning the conductor layer by wet-etching
Thirdly the present invention provides a manufacturing method for a multi-layer wiring board comprising the steps of: manufacturing a green sheet including the binder containing the acrylic resin having no polar group and the ceramics material in powder; forming an interlayer connecting via through the green sheet in the width direction; after having placed a conductor layer having virtually no voids on the green sheet and then a mask on the conductor layer, patterning the conductor layer by wet-etching; laminating a plurality of the green sheets; after having placed on either both sides or one side of the green sheet multi-layered body thus manufactured a binding sheet containing an inorganic composition that has virtually no sintering shrinkage at the firing temperature of the green sheet multi-layered body as a main component, allowing the resulting multi-layered body to undergo a firing process and then removing the binding sheet therefrom.
Fourthly the present invention provides a manufacturing method for a double-sided wiring board comprising the steps of: manufacturing a green sheet including the binder containing the acrylic resin having no polar group and the ceramics material in powder; forming an interlayer connecting via through the green sheet in the width direction; after having placed a conductor layer having virtually no voids on the green sheet and then a mask on the conductor layer, patterning the conductor layer by wet-etching; after having placed on either both sides or one side of the green sheet a binding sheet containing an inorganic composition that has virtually no sintering shrinkage at the firing temperature of the green sheet as a main component, allowing the resulting green sheet to undergo a firing process and then removing the binding sheet therefrom.
With the above-mentioned arrangement of the present invention, a preferable water resistant property is provided by the acrylic resin having no polar group that is contained therein; therefore, even if the subtractive method in which the green sheet is immersed in an etchant together with the conductor so as to form a wiring pattern is adopted, no damage is caused in the green sheet. Thus, it becomes possible to positively form a wiring pattern made of metal foil on the green sheet.
Moreover, in the case when the conductor layer made of a conductor having virtually no voids is placed on the surface of the sheet, it is not necessary to apply a binder-eliminating process to the conductor layer. This makes it possible to prevent the generation of bubbles that would otherwise be caused by the binder-eliminating process, and also to prevent shrinkage in the X-Y direction; therefore, the pattern (wiring pattern) of the conductor layer, formed on the green sheet, can be maintained as it is. As a result, it becomes possible to provide wiring having a low resistivity that would not be achieved by wiring that uses conductor paste.
The binder preferably contains a methacrylic-acid-based acrylic resin as a main component. With this arrangement, since the binder is made from a methacrylic-acid-based acrylic resin as its main component, a better thermal decomposing property is provided so that it is possible to carry out a binder-eliminating process in a non-oxidizing atmosphere. Thus, it becomes possible to realize a simultaneous firing process together with, for example, copper conductor wiring.
Moreover, the binder may preferably contain the acrylic resin having no polar group at a weight rate of not less than 10% in its blending ratio. This arrangement allows the binder to exert a better water resistant property so that it can sufficiently withstand the immersing process to an etchant.
The above-mentioned binder may also contain the acrylic resin having no polar group at a weight rate of not more than 50% in its blending ratio. This arrangement allows the binder to exert sufficient sheet strength as a base material.
Moreover, the conductor layer is preferably made of metal foil. This arrangement makes it possible to form a conductor layer having virtually no voids more easily.
With respect to the above-mentioned conductor layer, the metal foil is preferably subjected to a roughening process on its surface. This arrangement makes it possible to further strengthen the joining between the conductor layer and the green sheet.
Moreover, it is preferable to place on the conductor layer a mask containing an inorganic composition that has virtually no sintering shrinkage at the firing temperature of the green sheet as its main component. With this arrangement, the mask containing the inorganic composition as its main component can be easily removed after the firing process; therefore, it becomes possible to simplify the mask separation process that is carried out after the formation of wiring through a patterning process on the conductor layer by means of etching. In the case when this green sheet is used for forming a double-sided or a multi-layer wiring board, the effectiveness of such an advantage can be realized when forming the wiring on the double-sided wiring board or on the outermost layer of the multi-layer wiring board.
With respect to the etchant used in the wet-etching, it is preferable to use a water solution of ferric chloride; thus, in the case of the conductor layer made of copper, it becomes possible to form a pattern at high speeds without causing any damages to the green sheet.
With respect to the mask, it is preferable to use virtually the same material as that of the binding sheet. This arrangement makes it possible to simultaneously remove the mask formed as the uppermost layer at the time of removing the binding sheet. Consequently, it is possible to reduce the number of processes by eliminating the removing process for the mask as the uppermost film.
With respect to the inorganic composition contained in the binding sheet and the mask, Al2O3 is preferably used.